Two-sided lighting, rename misleading draw-count stat

Two bugs conflated as "weird colors":

1. Two-sided lighting.  IFC placements often embed reflection
   matrices (mirrored families).  Transforming a_normal by
   mat3(inst.transform) produces a normal pointing the wrong way
   on those instances, and max(n·L, 0) then clamps the surface to
   pure ambient — reads as dark / washed out.  Use gl_FrontFacing
   to flip n in the fragment shader so both winding orientations
   shade correctly.  The proper fix (ship an inverse-transpose
   normal matrix or a det-sign bit per instance) is still owed;
   that would unlock re-enabling GL_CULL_FACE for a big fragment-
   work win on closed solids.

2. Stats label "inst_draws" was counting indirect sub-draws, not
   actual GL draw calls — misleading since MDI collapses N sub-
   draws into one glMultiDrawElementsIndirect.  Split into
   gl_draw_calls (real GL calls, = drawn-model count) and
   indirect_sub_draws (packed sub-commands).  For a BIM model
   with 47k unique meshes at full view this now correctly reads
   "1 gl_draws (47092 sub)" rather than suggesting 47k driver
   dispatches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ifcviewer/ViewportWindow.h

@@ -136,7 +136,8 @@ public:
         uint32_t total_triangles;
         uint32_t visible_triangles;
         uint32_t unique_meshes;
-        uint32_t instanced_draws;
+        uint32_t gl_draw_calls;        // actual glMultiDrawElementsIndirect issues per frame
+        uint32_t indirect_sub_draws;   // total commands packed into those indirect buffers
     };
 
 signals:
@@ -202,7 +203,8 @@ private:
     // Per-frame stats
     uint32_t visible_triangles_ = 0;
     uint32_t visible_objects_ = 0;
-    uint32_t instanced_draws_ = 0;
+    uint32_t gl_draw_calls_ = 0;
+    uint32_t indirect_sub_draws_ = 0;
 
     // Reused scratch: visible-instance index lists per mesh, flattened into
     // `visible_flat_` for upload.  Both live in the parent object to avoid